The present invention relates to optical transducers (sometimes referred to as optical sensors) which utilize optical means for sensing mechanical displacements, such as movements of a body or deformation of a membrane, and converting them to electrical signals. The invention also relates to methods of making such optical transducers. The invention is particularly useful in making optical microphones, for converting sound into electrical signals, and is therefore described below especially with respect to this application.
Optical transducers of this type are described, for example, in U.S. Pat. Nos. 5,771,091; 5,969,838; 6,091,497; and 6,239,865, the contents of which patents are incorporated herein by reference.
Such optical transducers generally include an optical unit containing a light source, a laterally spaced light detector, an optical shield between the light source and light detector, and a displaceable member aligned with the optical window defined by the light source, light detector, and optical shield between them. The displaceable members in the optical transducers described in the above patents are generally in the form of deformable membranes, but may be physically movable members, such as in an accelerometer. Optical transducers of this type can be constructed to be very accurate for measuring very small displacements.
In general, the known optical transducers do not readily lend themselves to low-cost mass production, and therefore are relatively expense to produce. One particularly troublesome problem for low-cost mass production of such optical transducers is the need to have the optical shield extremely thin and precisely located with respect to the light source and light detector. An extremely thin optical shield is needed in order to minimize shading and maximize the effective surface of the light source and light detector exposed in the optical window to produce a maximum sensitivity to the displacements of the displaceable member. In the prior art constructions, such an optical shield was generally provided by using a thin metal foil, e.g., of aluminum but the introduction of such a thin metal foil does not lend itself to mass-production or automated processes.
One object of the present invention is to provide a method of producing optical transducers in a manner which permits mass production at relatively low cost. Another object of the invention is to provide a novel optical transducer of an efficient construction which maximizes the effective surfaces of the light source and light detector exposed in the optical window of the transducer.
According to one aspect of the present invention, there is provided an optical transducer, comprising: a base member; a light source carried on a face of the base member; a light detector carried on the face of the base member laterally spaced from the light source; an optical shield extending from the face of the base member between the light source and light detector; and a displaceable member overlying, and spaced from, the light source, light detector and optical shield, and effective to reflect light from the light source to the light detector; the optical shield being constituted of a transparent plate extending from the face of the base member between the light source and light detector and carrying a thin layer of a light-blocking material to shield the light detector from direct exposure to the light source.
As will be described more particularly below, such an optical transducer readily lends itself to mass-production. It also minimizes the shading effect of the light detector and maximizes the effective surface of the light source and light detector exposed in the optical window, thereby improving the sensitivity of the optical transducer to the displacements of the displaceable member.
According to further features in the preferred embodiment of the invention described below, the transparent plate of the optical shield is fixed within a groove formed in the face of the base member between the light source and light detector. Preferably, the groove is less than one-half the thickness of the base member. The light source, light detector, and optical shield are all embedded in a transparent plastic potting material.
According to another aspect of the present invention, there is provided a method of making an optical transducer comprising: providing a light source and a light detector on a face of a base member, with the light detector spaced laterally from the light source; and fixing to the face of the base member, between the light source and light detector, an optical shield including a transparent plate carrying a thin layer of a light-blocking material to extend between the light source and light detector, and thereby to shield the light detector from direct exposure to the light source. The light source, light detector and optical shield are then embedded in a transparent plastic potting material.
According to a still further aspect of the present invention, there is provided a method of making optical transducers, comprising: applying to a face of a base member a plurality of pairs of light sources and light detectors, with each light detector spaced laterally of the light source of the respective pair; fixing to the face of the base member an elongated strip of transparent material extending between the light sources and light detectors of all the pairs, the transparent strip carrying a thin layer of a light-blocking material on one of its sides; embedding a transparent material over said light sources, said light detectors and said transparent strip with light blocking material on one of its side and cutting the base member into individual optical units each including a light source, a light detector spaced laterally from the light source of the respective pair, and a section of the elongated strip, the section constituting an optical shield between the light source and light detector of the respective pair to shield the respective light detector from direct exposure to the light source of the respective pair.
The plurality of light sources, light detectors and elongated strips are embedded in a transparent plastic potting material before the base member is cut into the individual optical units.
According to still further features in the described preferred embodiment, a deformable membrane is mounted to the outer end of the transparent plastic potting material to overlie, and to be spaced from, the light source, light detector and optical shield.
Preferably, the base member is a printed circuit board which includes the electrical connections to the light sources and light detectors.
As will be described more particularly below, the foregoing features not only permit the optical units of the transducers to be produced in volume and at low cost, but also provide an efficient construction maximizing the effective surfaces of the light source and light detector exposed to the displaceable member.